1. Field of the Invention
The invention relates to fluid flow meters and more particularly to flow meters which incorporate independently counter rotating turbine impellers in the sensing mechanism.
2. Description of the Prior Art
A number of fluid flow meters utilizing rotating turbine impellers are known in the prior art. In these devices, the meter is placed in the line through which the flow is to be measured and the angular velocity of the turbine, driven by the fluid flow, is measured and correlated to the flow rate.
In prior art meters of this type, upstream disturbances in the flow pattern produced a non-linear rotational velocity as related to actual flow rate. This non-linearity reduced the reliability and usefulness of these meters or required additional electronic compensation to achieve an acceptable level of accuracy and range.
Also disclosed in the prior art are several designs in which an additional turbine impeller or turbine shaped stator is interposed in the flow path within the meter. One such prior art device, disclosed in U.S. Pat. No. 3,241,366, adds a counter rotating impeller or rotor upstream from the measuring rotor. This upstream rotor functions only to direct the fluid flow on the blades of the principal or measuring rotor. This upstream rotor termed a "compensating rotor," requires a magnet and pins to effect a magnetic drag on the compensating rotor which can be adjusted to extend the accuracy of registration of the measuring rotor over an extended range of flow rates.
In this disclosure, only the downstream rotor is a measuring rotor and its performance curves will be substantially similar to those disclosed and illustrated hereinafter for a downstream rotor or impeller. The compensating rotor simply extends the measuring range for conditions of low flow. The disclosure of the present invention in which both impellers are measuring impellers, provides a simpler construction which will achieve measurements at least equally reliable and further includes self diagnostic means for early detection of a malfunction.
Also, U.S. Pat. No. 3,135,116 reveals a dual turbine design in which both rotors rotate in the same direction, one mounted piggyback fashion on the rotating shaft of the other. A flow straightener is interposed between the two turbines which acts, in contrast to the present device, to fluid dynamically decouple the rotors.
U.S. Pat. No. 138,657 discloses a meter having a dual augershaped screw arrangement in which the upstream screw is stationary and the forward or downstream screw revolves in response to flow through the meter.
U.S. Pat. No. 470,814 shows a similar arrangement including one fixed and one rotating turbine - shaped deviced in which the blades of each are angularly opposed to one another.
U.S. Pat. No. 3,710,622 discloses a dual rotor meter in which the rotors have different parameters, rotate in the same direction and are not fluid dynamically coupled.
The device described herein will be seen to be different in concept and operation from the above or any other presently known to the inventor.